This invention relates to a CDMA (Code Division Multiple Access) receiver and, in particular, to a multi-path detection method and circuit that is used in the CDMA receiver.
Conventionally, a wide variety of cellular communication systems have been proposed and used in the world. Among others, recent attention has been focused onto a cellular mobile CDMA system that has a specific spread code assigned to each channel and that will be simply called a CDMA system hereinafter. In such a CDMA system, a modulated radio wave is generated by spreading a transmission signal by the specific spread code to obtain a spread signal and by modulating the same carrier frequency by the spread signal and is transmitted from a transmission side (transmission terminal) to a reception one (reception terminal).
Responsive to the modulated radio wave, a CDMA receiver on the reception side executes synchronizing operation by the use of each of the specific spread codes and identifies a desired channel. From this fact, it is readily understood that each radio channel between a base station and a mobile terminal can be distinguished from one another by using different spread codes. At any rate, the CDMA system makes it possible to use the same carrier frequency by a plurality of stations (base station and mobile terminal) and to identify each station by the use of the spread codes.
In this event, it is to be noted that the modulated radio wave is received by the reception side through a plurality of transmission or propagation paths that may be collectively called a multi-path. Therefore, multi-path fading should be removed from the modulated radio signal by correctly detecting a predetermined signal, such as a synchronization signal and/or a pilot signal, in the CDMA system. In other words, a transmission signal transmitted from a transmission station in the CDMA system is reflected by buildings, mountains, and the like, propagated through a plurality of transmission paths that are subtly different in propagation time from one another, and thereafter received by a reception station as reception signals which are usually referred to as a multi-path signal. This shows that the reception station should receive the multi-path signal in timed relation to each timing of the reception signals.
In Japanese Unexamined Patent Publication No. Hei 9-181704 (181704/1997) (will be called first reference), description is made about a conventional CDMA signal receiver apparatus and a multi-path searching method. Specifically, the CDMA signal receiver apparatus described in the first reference has a first portion for carrying out multi-path detection in consideration of a communication environment and a second portion (rake combining portion) for combining a plurality of paths in phase with one another. The first portion is called a multi-path search portion or a searcher while the second portion is called a rake combining receiver or a rake receiver.
Herein, the above-mentioned CDMA signal receiver apparatus operates in a following manner. At first, the multi-path search portion measures a delay profile to select some paths that are large in reception power within a measurement range and to inform the rake receiver of timing of each of the paths. The “delay profile” means a signal power distribution relative to a delay time. The rake receiver carries out de-spread operation of each path on the basis of the timing information sent from the multi-path search portion and executes rake combining operation. The resultant rake receiver can realize a path diversity effect.
In the meanwhile, the rake receiver in the first reference has a tracking portion that tracks or follows a variation of a designated path. In this case, path information should be informed from the multi-path search portion to the rake receiver at least at an initial period or at a predetermined period.
The first reference has two problems. One of the problems is that, in order to search some paths from multi-paths with reference to the measured delay profile, maximum search operations should be executed by the number of the searched paths or sort processing should be executed over whole profile data. Consequently it takes a long time to search the multi-path, which brings about an increase of current consumption.
In addition, an optimum device should be prepared so as to search a desired number of correlation peaks from the delay profile. This is because the delay profile generally includes a great amount of data and a processing time inevitably becomes long so as to search a plurality of peaks from the whole data of the delay profile.
Another one of the problems described in the first reference will be described in detail. In general, a configuration of the correlation peaks appearing on the delay profile has widths along a time. Therefore, a sort algorithm of the binary tree search is disadvantageous in that data included in a single peak are repeatedly detected. This shows that the first problem can not be always solved even when usual high speed sort algorithm is used.
In this case, a method is used to calculate a minimum interval of each finger position by removing, from a next following peak detection, a detected peak and t-samples prior and after the detected path and to set the minimum interval of each finger position (For example, Aoyama et al. “Path-Search Performance of DS-WCDMA System in Laboratory and Field Experiments” Technical Report of IEICE, RCS97-164 (1997-11)). However, it is required in this method to repeat, over the number of the detected peaks, various processes, such as a maximum value search, a removal of t-samples prior and after the peak. Therefore, the multi-path detection circuit mentioned in the first reference is disadvantageous in that a processing time becomes long as an increase of the peak number of the detected peaks.
A similar rake receiver of the type described is known also in any other references. For example, disclosure is made about such a rake receiver in Japanese Unexamined Patent Publication No. 2000-4211 (will be called second reference). Simply, the rake receiver disclosed in the second reference can keep synchronization by a small amount of processing. To this end, a plurality of de-spreaders are prepared in each finger receiving circuit so that multi-path components can be received even when the delay time in the multi-path component is largely varied in an environment. In other words, the rake receiver can receive the multi-path components even when any tracking can not be achieved by synchronization operation, such as synchronization capturing operation and synchronization keeping operation which uses DLL (Delay Locked Loop) technique. Specifically, the multi-path components can be received by the de-spreaders at different timing while a selector instantaneously selects an output signal from either one of the de-spreaders at timing corresponding to the varied delay time. As a result, each finger receiver can receive the multi-path component in a good condition and the rake receiver can receive an excellent desired wave.
As mentioned above, each finger receiver described in the second reference comprises the plurality of the de-spread circuits for demodulating the reception signal by using the spread codes that are shifted by times different from one another in the delay circuits and the selector responsive to outputs given from the de-spread circuits for selecting either one of output signals that has a good quality. However, no consideration is made at all about a multi-path detection circuit and its structure.
In Japanese Unexamined Patent Publication No. 2000-244456 (will be called third reference), disclosure is made about a path detection device and its control method of detecting a delayed wave received through a long delay path in a DS(direct sequence)-CDMA demodulator. In the path detection device mentioned in the third reference, when a reception signal sampled is given to a matched filter, the sampled reception signal is supplied at every sample to a shift register of the matched filter. The signals given to the shift register are multiplied through a switch by spread code replicas stored in registers, respectively, to obtain products or results of multiplication. The products are added by adders to one another to calculate a correlation value. The calculated correlation value is output from the matched filter.
Thus, the third reference discloses a technique of forming a plurality of delay profiles by the matched filter in order to detect a path of a delayed wave having a long delay time. However, no teaching is made in the third reference about executing multi-path detection at a high speed.
Moreover, description is made in Japanese Unexamined Patent Publication No. 2000-252867 (will be called fourth reference) about a spread spectrum communication device that can accomplish synchronization of a spread code at a high speed by shortening a time necessary for searching a multi-path. More specifically, the spread spectrum communication device described in the fourth reference has a plurality of correlators for carrying out rake receiving operation and a code generation timing controller. With this structure, each correlator gives generation timing of a spread code for carrying out de-spreading operation about a maximum path during reception and the code generation timing controller estimates a time region during which a multi-path signal is received with a high probability. In addition, the code generation timing controller controls generation timing of a spread code replica so as to receive either one of the correlators that operates as a searcher within the above-mentioned time region.
The fourth reference thus describes about shortening a time for a multi-path search. In the fourth reference, estimation is done about the time region wherein the multi-path signal is received with a high probability. Such estimation is carried out on the basis of generation timing of the spread code produced for de-spreading processing and a result of estimation is used to control the generation timing of the spread code replica in the searcher.
Furthermore, Japanese Unexamined Patent Publication No. Hei 10-271557 (namely, 271557/1998) (will be called fifth reference) discloses a random access signal receiver which is operable in a random access mode. In this random access signal receive, a time shortage for estimating a delay profile and for training necessary for path detection can be avoided together with restriction of an amount of control information transmitted. Specifically, the random access signal receiver comprises a delay profile timing detector, a matched filter and a path extractor. With this structure, the delay profile timing detector supplies path detection timing to the matched filter and the matched filter produces a de-spread signal on the basis of the path detection timing. The path extractor latches the de-spread signal sent from the matched filter and produces a transmission symbol of each path corresponding to each de-spread signal. Moreover, the transmission symbol of each path is detected by each detector and is converted into binary data in each data detection portion. Each random access judgment portion is operable to detect a correlation between the binary data and a random access starting synchronization word and to allow the binary data to pass therethrough as control information on detection of the above correlation. Finally, the random access judgment portion stops generating the control information when detection is made about a correlation between the binary data and a random access end synchronization word.
This shows that the fifth reference discloses a technique of independently receiving the random access signal from a plurality of mobile stations at every path.
In addition, Japanese Patent Publication No. 2765574 (will be called sixth reference) discloses a CDMA chip synchronization circuit that is used in a mobile communication system of a wideband CDMA type and that enables improvement of reception quality in a wideband CDMA receive and high speed transmission. Such improvement of the reception quality and the high speed transmission can be achieved by securely detecting or tracking multi-path timing within a low Eb/No environment. More specifically, the CDMA chip synchronization circuit described in the sixth reference has a search portion for detecting reception timing and a correlator for carry out correlation calculation (de-spread operation) at predetermined timing that may be shifted, for example, at one-fourth (¼) time interval with reference to the reception timing. Thereafter, selection is made of a timing signal that gives a most excellent reception quality after the de-spread operation. This structure enables reliable reception even when the reception timing is discontinuously or irregularly varied. In addition, this structure is different from DLL in the viewpoint that timing is securely captured in a peak of a reception level in a propagation environment such that a plurality of paths are not completely separated from each other and consequently are received in a superposed manner.
As mentioned before, the six reference describes about a technique of detecting reception timing by a searcher, obtaining timing shifted by a predetermined time interval shorter than a single chip period on the basis of the reception timing, simultaneously executing a correlation calculation (de-spreading), and thereafter selecting a signal that is received at timing at which reception quality is most excellent after the de-spreading. However, no teaching is directed in the sixth reference to the multi-path circuit, namely, the searcher.